The present invention relates to a protected optical disc used for the recording of information in the form of video or digital signals.
It is known that in order to obtain a high information density on a relatively compact carrier it is advantageous to use an optical disc having at least one layer, whose physical characteristics vary as soon as it is exposed to radiation in which the information is recorded by means of a focused light beam in the form of diffracting microelements, which can then be detected by means of another focused light beam for the purpose of reading the information. In order to permit easy, frequent manipulations by a user the commercially sold pre-inscribed or blank disc must have a protection to obviate any influence of dust, finger marks, etc., on the disc and also to prevent any deterioration of the radiation-sensitive layer.
The manufacture of pre-inscribed discs is based on the pressing of a transparent support from a matrix carrying the information to be reproduced on a large number of discs. The manufacturing operations are performed in a dust-free atmosphere, great care being taken to ensure that the optical support is not deformed. The discs obtained carry an optically legible information, but they cannot be recorded by the user.
It is much more complicated to produce discs which can be inscribed by the user, because it is necessary that the protection added to the layer used for recording the information does not disturb the passage of an information-modulated focused light beam. Furthermore the recording procedure must not require development. Inscribable discs are known which comprise a layer which undergoes a permanent modification under the action of an inscription radiation placed between two plates, one of which is transparent, the layer being deposited on one of the plates. The two plates are separated by shims in order to provide a free space in front of the layer to permit the giving off of decomposition products, for example metal vapours when the layer is a volatilisable metal layer. The need to make shims with a strictly constant thickness so as not to destroy the flatness of the plates, which must themselves be sufficiently thick and rigid to ensure a good protection and the assembly of the whole unit leads to high production costs.